1. Field of the Invention
The present invention relates to a magneto-optical recording medium and, more specifically, to a material of a thin film containing at least a magneto-optical recording film laminated on one side of a transparent substrate and to the laminated structure.
2. Discussion of Related Art
A data recording medium using a magneto-optical recording medium is now at the stage of practical use, and thus it is an increasingly important technical problem to provide a magneto-optical recording medium which has an overwrite function, a higher reproduction CN ratio, and further an excellent property for preserving data for a long time, the overwrite function being a function, by which, when new data is to be written to an area to which previous data has been recorded, the new data can be simultaneously written while the previously recorded data is being deleted or without the need for erasing the previously recorded data, instead of that whereby the new data is written after the previously recorded data has been erased once.
Fundamentally, the magneto-optical recording medium can be made by providing a magneto-optical recording film on one side of a transparent substrate, the magneto-optical recording film being composed of, for example, a rare earth metal-transition metal type amorphous vertically-magnetized film, Pt-Mn-Sb type alloy (Heusler alloy) film, laminated substance of a Pt film and Co film, Mn-Bi type alloy film (crystalline substance), oxide film, and the like which can thermomagnetically carry out recording and erasing, and magneto-optically reproduce recorded signals when irradiated by a laser beam. The above various characteristics, however, cannot be realized only by the fundamental arrangement and the arrangement cannot be practically used. Thus, various technologies have been conventionally proposed to provide the above various characteristics.
As described in, for example, Japanese Patent Application (Laid-Open) No. Sho 63-241739, technology for providing an overwrite function with a magneto-optical recording medium is such that a ferrimagnetic thin film having a compensating temperature higher than room temperature is used as a magneto-optical recording film and an in-plane magnetized layer, the magnetizing direction of which is adjusted to be parallel with the film surface of the ferrimagnetic thin film, is formed on the surface thereof to restrict the generation of a Bloch line so that data can be overwritten by an optical modulation system.
As described in, for example, Japanese Patent Application (Laid-Open) No. Sho 57-169996, technology for improving a reproduction CN ratio is such that an enhanced layer composed of an inorganic dielectric substance having an index of refraction higher than that of a transparent substrate, magneto-optical recording layer, protective layer composed of a material similar to that of the above enhanced layer, and reflection layer composed of a metal material having a high reflectance are each laminated on one side of the transparent substance sequentially from the transparent substrate side. According to the magneto-optical recording medium, since the enhanced layer is provided on the boundary between the transparent substrate and the magneto-optical layer, an optical beam for reproduction is multi-reflected between the transparent substrate and the magneto-optical recording layer so that the light beam for reproduction is multi-reflected between the transparent substrate and the magneto-optical recording layer to thereby increase the apparent Kerr rotation angle of the polarization rotation of a linearly polarized beam reflected off the film. Further, since the protective layer and reflection layer are sequentially laminated on the backside of the magneto-optical recording film viewed from the transparent substrate side, the optical beam for reproduction passing through the magneto-optical recording film can be reflected in the reflection layer and returned to the transparent substrate side, so that an apparent Kerr rotation angle of the polarization rotation of a linearly polarized beam reflected off the film can be further increased by the Faraday effect received by the optical beam when it passes through the magneto-optical layer (reciprocating process). Since the reproduction CN ratio is in proportion to a product of a Kerr rotation angle and reflectance, the magneto-optical recording medium can provide a high reproduction CN ratio.
Alternately, as described in, for example, Japanese Patent Application (Laid-Open) No. Sho 61-188762, another example for improving a reproduction CN ratio of a magneto-optical recording medium is such that an in-plane magnetized layer is provided on the boundary between a transparent substrate and a rare earth metal-transition metal type amorphous vertically-magnetized film as a magneto-optical recording film and on the surface of the above amorphous vertically-magnetized film, and the thickness of the amorphous vertically magnetized film and in-plane magnetized layer is specified to increase a Kerr rotation angle and the rectangularity of a Kerr hysteresis.
As described in, for example, Japanese Patent Application (Laid-Open) No. Sho 60-231935, a technology for improving the data preserving property of a magneto-optical recording medium is such that a rare earth metal-transition metal type amorphous vertically-magnetized film as a magneto-optical recording film is provided on a transparent substrate and an in-plane magnetized layer composed of a rare earth metal or transition metal similar to the above or an alloy of them and a protective layer composed of silicon nitride are sequentially formed on the surface of the amorphous vertically-magnetized film to prevent the oxidization and deterioration of the amorphous vertically-magnetized film.
Nevertheless, these conventional proposals do not take into consideration at all the effect of heat applied by a laser beam focusing on a magneto-optical recording film when data is recorded, reproduced or erased. In particular, since they do not take into consideration the problem of thermal diffusion and thermal capacity in a thin film provided in contact with the magneto-optical recording film (which is referred to as an adjacent film), the following technical problems arise:
(1) When the thermal diffusion in the adjacent film is too late or when the thermal capacity thereof is too large, a large laser power is required to record and erase data and thus a drive unit becomes expensive;
(2) When the thermal diffusion of the adjacent film is too late, much heat is accumulated in the portion where a laser spot is irradiated and thus a change, such as a structural easing, crystallization and the like is liable to be made to the magneto-optical recording film. Further, although a preformat pattern for optically reading out data is formed in a fine irregular shape on a transparent substrate where a thin film is formed, the preformat pattern is liable to be thermally deformed by the heat produced when a laser beam is irradiated. Consequently, as data is repeatedly recorded, reproduced and erased, the level of a reproduced signal is gradually lowered and the reproduction of data becomes impossible within a relatively short period;
(3) When the thermal capacity of the adjacent film is too small and the thermal diffusion therein is too late, a problem is liable to arise in that reproduction output is varied, recorded data is destroyed by the change of an atmospheric temperature, and the like in addition to the disadvantage of the above item (2);
(4) Since a conventional medium needs a large external magnetic field for recording and erasing, overwrite using a magnetic modulation system is impossible, because a large magnetic field is difficult to be switched at a high speed. Therefore, a medium requiring a small magnetic field for recording and erasing is needed.